By Sam Wilson BBC News website

Arsonists may be motivated by excitement, revenge, or profit

As Greece battles against the most deadly and devastating forest fires for decades, the prime minister has expressed anger that some may have been started deliberately.

Costas Karamanlis said so many fires had broken out within hours, that it "cannot be a coincidence".

There were at least three arrests of people suspected of arson, and reports of cotton wool found soaked in petrol. The suspects included a man in his 60s, as well as two youths.

At least 50 people have been killed by the blazes, while great swathes of countryside and scores of homes have been consumed.

While lightning and other environmental factors are responsible for some forest fires, many are started by humans.

'Hero factor'

Many are down to careless accidents - a discarded cigarette or neglected campfire - but a large number, according to experts, are started in cold blood.

"Generally about 30% of human-caused wildfires are deliberately set," says Timothy Huff, a former FBI profiler specialising in arson, who has investigated hundreds of fires in California and elsewhere.

Investigators say developers may start fires to clear land

Mr Huff lists six potential motives for arsonists: revenge, excitement, profit, vandalism, extremism, and to cover another crime.

"Excitement is one of the big ones for wildfires," he told the BBC News website.

This covers not only teenaged boys, getting a thrill by watching a fire take hold and spread, but those motivated by the "hero factor" - people who want to be involved and to take the credit for fighting the blaze.

Also "revenge sometimes figures", says Mr Huff.

"And here there's the overriding factor of power - that is, the power to get revenge against a person that's offended them, or a group or institution, or society in general.

"These are set by persons who believe they've got a bum deal and want to get back against the world."

Profit motive

While there is no suggestion that this is the case in Greece, suspicion sometimes falls on firefighters themselves.

After very destructive wildfires that swept Spain and Portugal two years ago, a senior Spanish firefighter was accused of starting them in a fit of pique at not being assigned to a local brigade.

There may be another incentive for part-time or retained firemen to start the very thing they are supposed to stop - profit.

"As long as there are people who make a living from fires, there will be an incentive to set wildfires," forest engineer Ricardo Terra Santos, who has co-authored a book on the state of Portugal's forests, told the AFP news agency.

It is harder to work out who started a fire than how it was started

That motive may also extend to firms that rent out fire-fighting equipment, hunters who want to clear brush, or property developers frustrated by environmental protection for forests.

Where, as in Greece, laws say forested land cannot be built on, some developers may decide on a logical but drastic course of action.

'Crime of stealth'

Investigators can often get an idea of how arson was committed, but identifying and prosecuting the culprit is much harder.

"Arson in general is very difficult to prosecute, because it's a crime of stealth," says Alan Clark, executive director of the International Association of Arson Investigators.

"With wildland fires there are usually not witnesses and they take place in remote areas. So the detection rate is not so high."

The deadly risk of starting a fire may seem obvious to some, but Mr Huff says the arsonist often does not intend to endanger life.

Elderly homeowners, children and firefighters are among those to have perished in Greece's fires.